1. Field of Invention
This invention relates to angular sensors and more particularly relates to angular sensors used to detect steering wheel position, especially a steering angle sensor used for yaw angle control in an automotive vehicle.
2. Description of Related Art
In control systems for yaw angle control of an automobile, it is important that a value describing the angle position of the front wheels is in-put. However, for practical reasons it has become usual not to directly measure the steering angle of the wheels but rather the angle of rotation of the steering wheel which is closely associated with the angle position of the front wheels. To accomplish this, there are sensors which work with a so-called code disk operatively coupled with the steering wheel. Such a device has been described in the applicant's patent application 185 32 903.1. To effect a finer adjustment of the desired angle position of the wheels and to reduce the energy effort needed for steering, however, a transmission is inserted between the steering wheel and the vehicle wheels so that a plurality of revolutions of the steering wheel is necessary for adjusting the vehicle wheels by a larger angle.
Since the above-mentioned code disk is coupled with the steering wheel, it is possible to indicate the absolute angle position of the steering wheel, yet not how many revolutions of the steering wheel have already taken place, starting from a neutral position of the vehicle wheels.
One solution is to measure the difference in the speeds of rotation of the front wheels of a vehicle during driving. This allows for a rough determination of the angle position of the front wheels so that it is possible to determine the number of revolutions already performed by the steering wheel, starting from a normal position. In this manner, permanently adding the steering wheel revolutions in one direction or in the other direction, it is possible to follow the number of the momentarily available steering wheel revolutions and to thus provide corresponding information. This is, however, impossible for modern steering angle sensors with electric or electronic evaluation such as in the case of the above-described code disk since, during parking, the electric power of these vehicles is turned off while it must nevertheless be possible to pivot the wheels in the turned-off condition. When turning on the electric power again, in this case, the relevant value of revolution would actually differ from the value stored at the time when the electric power supply was turned off. Additionally, the determination of the number of revolutions becomes quite inaccurate during extremely slow speeds due to different speeds of rotation of the front wheels.